ev is the "visual critical corona voltage" or "corona inception voltage" (CIV), the voltage required to initiate a visible corona discharge between the wires.
mv is an irregularity factor to account for the condition of the wires. For smooth, polished wires, mv = 1. For roughened, dirty or weathered wires, 0.98 to 0.93, and for cables, 0.87 to 0.83, namely the surface irregularities result in diminishing the corona threshold voltage.
r is the radius of the wires in cm.
S is the distance between the center of the wires.
gv is the "visual critical" electric field, and is given by:
g0 is the "disruptive electric field."
c is an empirical dimensional constant.
- The values for the last two parameters are usually considered to be about 30-32 kV/cm (in air ) and 0.301 cm½ respectively. This latter law can be considered to hold also in different setups, where the corresponding voltage is different due to geometric reasons.
Note: The formula has an unintuitive implication that "if the distance between the wires is half the diameter (the radius) then the voltage needed for discharge is zero. Ln (1) = 0." I conclude that the distance is measured from the center of the wires. Not obvious in the definition of "S". Thank you.
- Hong, Alice (2000). "Electric Field to Produce Spark in Air (Dielectric Breakdown)". The Physics Factbook.